Dimensions | Citadel docs

dimensions:
  dungeon:                          # castle:dungeon, saved to world_dungeon
    type: { has_skylight: false, has_ceiling: true, weather: false }
    world: script                   # or void

local dungeon = world.dimension("dungeon")

that world's world table; "minecraft:the_nether" works too

dungeon.on_generate(function(chunk) chunk:fill(chunk.min, chunk.min + vec3(15, 0, 15), "bedrock") end)

routes.add{ from = "overworld", to = "dungeon", trigger = { block = "light_blue_carpet", ticks = 1 }, placement = { x = 0.5, y = 65, z = 0.5 } }

routes.add{ from = "dungeon", to = "overworld" }

no trigger: only player:travel takes it

player:travel("dungeon", vec3(0.5, 65, 0.5))

to a point, or without one to its spawn

type fields, all optional and starting from the main world’s: has_skylight, has_ceiling, weather, ultrawarm, bed_works, respawn_anchor_works, coordinate_scale, fixed_time, effects.
A trigger block fires after ticks ticks standing in it (1 by default); after a trip the player steps out for half a second before it fires again.
A player’s inventory and health go with them; their file stays the main world’s. Dying anywhere respawns them in the main world.